Method and apparatus for controlling the degree of preparation of raw material for cement

ABSTRACT

Method and apparatus for controlling the degree of preparation of raw material for cement to be fed to a rotary kiln from a preheating zone comprising a series of ducts and separation chambers wherein, when the raw material has reached a temperature intermediate the decomposition temperatures of magnesium carbonate and calcium carbonate, the raw material is subjected to a rapid temperature drop capable of delaying calcium carbonate decomposition until the raw material has entered the rotary kiln.

United States Patent [191 Lippmann METHOD AND APPARATUS FOR CONTROLLINGTHE DEGREE OF PREPARATION OF RAW MATERIAL FOR CEMENT [75] Inventor: JeanPierre Lippmann, Soisy Sous Montmorency, France [73] Assignee:Creusot-Loire, Paris, France [22] Filed: July 29, 1971 [21] Appl. No.:167,294

[30] Foreign Application Priority Data [451 Sept. 25, 1973 2.66156112/1953 Muller 263/32 R 3.563519 2/1971 Lippmann 263/32 R FOREIGNPATENTS OR APPLICATIONS 1,154,012 8/1966 Great Britain 263/32 859.90710/1959 France 263/32 Primary ExaminerWi1liam F. ODea AssistantExaminer-Harold Joyce Attorney-William B. Kerkam, Jr. et a1.

[ 5 7 ABSTRACT Method and apparatus for controlling the degree ofpreparation of raw material for cement to be fed to a rotary kiln from apre-heating zone comprising a series of ducts and separation chamberswherein, when the raw material has reached a temperature intermediatethe decomposition temperatures of magnesium carbonate and calciumcarbonate, the raw material is subjected to a rapid temperature dropcapable of delaying calcium carbonate decomposition until the rawmaterial has entered the rotary'kiln.

13 Claims, 4 Drawing Figures METHOD AND APPARATUS FOR CONTROLLING THEDEGREE OF PREPARATION OF RAW MATERIAL FOR CEMENT This invention relatesto a method of controlling the degree of preparation of cement rawmaterials before entering the rotary kiln.

It is well known that the gases escaping from the rotary kiln ininstallations for the production of cement by the dry process are fed toan installation for preheating the material fed to the kiln. Thispre-heating installation may be constructed in various ways but usuallycomprises a series of heaters through which the hot gases leaving thekiln pass successively and in which the material is suspended in thecurrent of gas, each heater being followed by a zone for separation ofthe gas-material mixture, the material flowing from one separation zoneto another towards the kiln inlet and undergoing progressive partialdecomposition by heating in contact with the gases. In this way, thematerial fed at the top of the pre-heating installation has atemperature of about 800 to 850 on arriving in the kiln. Conversely, thegases leaving the kiln at a temperature of about 1,000 to 1,1 50 leavethe pre-heating installation end at a temperature of substantially 300".

A chute forming a continuation of the outlet orifice of the lastseparator delivers the materials heated in this wayeither directly tothe kiln inlet or to the inclined base of the kiln discharge gas boxwhich connects the kiln to the first discharge gas duct. It has beenfound that one of the gratest difficulties encountered in the use ofpre-heaters resulted from the clogging of the discharge gas box and thestart of said duct. Such clogging is always due to the condensation ofthe volatile impurities contained in the fuel and in the raw materials,which combine with one another to form harmful concretions ofsubstantially undefined chemical composition, generally mainly alkalineor alkaline-earth.

It has been found that these harmful condensations are practicallyalways due to the existence of too high a kiln discharge gastemperature, such temperature being greater than the condensationtemperature of such salts which are therefore deposited on the colderstatic parts of the installation with all the accompanying disadvantagesof clogging and instability of operation of the system. It is difficultto control the temperature of the rotary kiln discharge gas. Theinvention obviates these disadvantages and relates to a method ofcontrolling the degree of preparation of the material in the pre-heater,of modifying the heat exchanges between the material and the gasesinside the pre-heater, and thus indirectly controlling the discharge gastemperature.

It is further found that this problem can be avoided if the material issubjected-in the pre-heater zone in which it has reached a temperaturelevel between the decomposition temperatures of magnesium carbonate andof calcium carbonate to a rapid temperature drop capable of delaying thecalcium carbonate decomposition until after the material has entered therotary kiln.

Thus according to one aspect of the invention, there is provided amethod of controlling the degree of preparation of the raw materials forcement before such materials enter the rotary kiln, in a pre-heatinginstallation comprising a series of heaters through which the hot gasesleaving the kiln pass successively and in which the material issuspended in the current of gas, each heater being followed by a zonefor separation of the gas material mixture, the material flowing fromone separation zone to another towards the kiln inlet and undergoingprogressive partial carbonate decomposition by heating in contact withthe gases, characterized in that the material is subjectedin thepre-heater zone in which it has reached a temperature level between thedecomposition temperature of magnesium carbonate and the decompositiontemperature of calcium carbonateto a rapid temperature drop capable ofdelaying the calcium carbonate decomposition until after the materialhas entered the rotary kiln.

The invention also relates in a further aspect to apparatus forperforming the said method in conventional pre-heating installations.

The invention will now be described with reference to a number ofembodiments given by way of example and illustrated in the accompanyingdrawings, in which:

FIG. 1 is a general diagram of a conventional preheating installationprovided with a system adapted to performance of the method according tothe invention,

FIG. 2 is an elevation of a detail of a first embodiment of the systemaccording to the invention,

FIG. 3 is a plan view on the line III ill in FIG. 2, and

FIG. 4 is an elevation of a second embodiment of a system according tothe invention.

FIG. I diagrammatically illustrates a conventional installationcomprising a rotary kiln l, the discharge gases of which flow through adischarge gas box 2 to a material pre-heating installation comprising aseries of ducts, 31, 32, 33, 34 which interconnect material separationstages4l, 42, 43, 44 each comprising one or two cyclones, the materialoutlet orifice of each cyclone being extended by a tube 51, 52, 53,54for introducing the materials into the preceding duct (31, 32, 33, 34)or directly to the kiln inlet.

According to the embodiment shown in FIGS. 2 and 4, the end of eachmaterial supply tube is disposed on the axis of the gas flow duct. Thetube outlet may be closed by a balanced flap 6 (FIG. 2) comprising twocones 61 and 62 connected by their bases, the top cone 6] distributingthe material in a conical layer with the concavity extending downwardly,while the bottom cone 62 uniformly divides the ascending gas current togive a perfect distribution of the material in suspension in the gascurrent and thus improve the heat exchange between the material and thegas in the top part of the duct as far as the next separating cyclone.

It has been found that the material leaves the cyclone 42 at atemperature of about 600 and that it reaches a temperature greater than600 in the cyclone 41. The temperature of dissociation of the carbonatesof magnesium is known to be about 540 and that of calcium carbonate isabout 820. In these conditions, the magnesium carbonate whichpractically always accompanies calcium carbonate is completelydissociated when the material is introduced into the duct 31.

The existance of free magnesia appears to promote the formation of freelime. Consequently, the dissociation of calcium carbonate starts onlyafter dissociation of magnesium carbonate. For this reason, according tothe invention, the material is subjected to an abrupt temperature dropin the temperature range between the dissociation of its alkaline-earthsalts, i.e. between the last and last but one separation stage. Thisresults in a material quenching effect such that decomposition ofcalcium carbonate will be delayed until after the material has enteredthe kiln. in this way, the endothermic reaction for the dissociation ofcalcium carbonate will be shifted from cyclone 41 to the inlet of thekiln 1. This displacement of an endothermic zone is a means of modifyingthe heat exchange equilibrium in the kiln and in the pre-heater. Theshifting of the endothermic decomposition reaction to the kiln inletresults in a heat absorption and the discharge gases leaving the kilnare therefore colder. Since the calcium carbonate decomposition reactionrequires a certain temperature level and the presence of free magnesiaand since, after its abrupt cooling, the material encounters no furtherheat source until it enters the kiln, it is possible flexibly to controlthe temperature absorption at the kiln inlet by speeding up or reducingthe speed of the decomposition reaction in dependence on thetemperatures required at the kiln outlet. The delay in the calciumcarbonate decomposition can in fact be controlled by accurate control ofthe temperature of the mixture of gas and material in the region of thematerial distribution orifice, by controlling the cooling of thematerial.

On introduction into the duct 31, the material will preferably be cooledby an injection of cooled gas across the flow of material leaving thesupply tube. It will be apparent that it is possible to control thetemperature drop and hence the decomposition delay, by controlling thetemperature and the rate of injection of thecooling gases.

Asa result of this cooling of the kiln discharge gas exit end,thevolatile impurities contained in the kiln atmosphere can condense onthe materials arriving in the kiln or, on the kiln wall and will for themost part remain enclosed within the material under treatment and willthen be discharged to the exterior together with the clinker.

FIGS. 2 and 3 illustrate a first example of an embodiment of the'systemadapted to the performance of the method according to the invention.

FIG. 2 illustrates the top part of the gas flow duct 31 before its entryinto the cyclone 41, at the material supply tube outlet 52. Just belowthe flap or valve 6 the duct 31 and its refractory lining are formedwith a plurality of orifices 7 which flare out towards the interior ofthe duct and are uniformly distributed over the periphery thereof.

The orifices 7 are surrounded by an outer annular casing 8 into whichleads a cooling gas supply conduit 9. At the outlet of the conduit 9,the casing is provided with a deflector 10 which separates the currentof gas into two equal streams and the internal section of the casing 8decreases in size as far as the orifice 71 remote from the gas inlet, soas to maintain a constant pressure over the group of orifices 7. Thecold gases thus injected through orifices 7 are immediately taken up bythe hot streams of gases rising at high speed in the duct 31 andimmediately come into contact with the materials 11 dispersed in theform of layers by the cone 61, thus giving the required abrupt coolingof the materials.

In another embodiment shown in FIG. 4, the cooling gas supply conduit 9leads into an annular casing 12 surrounding the end of the materialsupply'tube 52. The casing 12 is closed at the top and is suspended fromthe tube 52. it is open at the bottom and the end of the tube 52 abovethe flap or valve 61 is provided with a conical deflector 63 whichproduces a layer of cooling gas which comes directly into contact withthe material already taken up and dispersed in the ascending current ofgas.

FIG. 4 also shows an example of a means of controlling the system.

The cooling gas supply conduit 9 is connected to the delivery of afresh-air intake fan 121 which is controlled by a variable speed motor13. By means of a thermocouple 15, a pyrometer 14 measures thetemperature of the gases leaving the kiln, i.e. upstream of thematerials dispersal orifice and the cooling system. The pyrometer 14 isset between two temperature levels 141, 142 forming the limits of theoptimum temperature of the kiln discharge gases. Between these twotemperatures, the pyrometer 14 through the agency of an amplifier 16 anda servomotor 17, controls the speed of the fan driving motor 13 betweenzero speed and maximum speed.

in this way, if the recorded temperature of the kiln discharge gas atthe thermocouple 15 is excessive, the pyrometer 14 controls the drive ofthe motor 13 which delivers the maximum gas 'flow into the casing 8 andcools the materials delivered by the flap 6. The calcium carbonatedecomposition reaction in the materials cooled in this way is thusdelayed and occurs in the kiln, the kiln discharge gas temperature beingreduced. As this temperature decreases to the optimum temperaturemeasured by the thermocouple 15, the speed of the fan and hence theinjected gas flow decreases. if the temperature drops below thepredetermined minimum temperature, the injection of cold gas is stoppedand the decomposition delay is thus reducedv until the temperature risesagain.

Of course the invention is not limited by the details of the abovedescribed exemplified embodiment, which could be modified withoutdeparting from the scope of the invention.

For example, to control the rate of flow of cooling gas it would bepossible to use a constant-speed fan provided with a control valve, theposition of which could be controlled by the temperature measured by thepyrometer 14, the valve being completely open at the maximum temperature141 and closed at the minimum temperature 142.

Instead of being external air, the cooling gas could be some of thegases which after passing through the preheater are taken from theoutlet thereof. It would also be possible to control the degree ofcooling of the material by controlling the temperature of the injectedgases; the gases taken from the pre-heater outlet at a temperature ofabout 300 as will have been apparent, may have been previously mixedwith cold external air introduced into the supply conduit 9. In such acase, the gas flows from the pre-heater and the external air would becontrolled by the measurement of the temperature of the kiln dischargegases, either by controlling the speed of the fans or the position ofthe control valves.

Although the installation has been described above in connection with acyclone pre-heater, it could readily be modified to adapt the method toany other type of pre-heater.

In prior art installations it was difficult to control the site of thecalcium carbonate decomposition reaction in the installation. With themethod according to the invention, by means of measurable cooling at apredetermined place in the pre-heater, it is possible reliably tolocalise the calcium carbonate decomposition endothermic reaction to thedischarge gas exit end of the rotary kiln and it is therefore easier tocalculate the heat exchanges in the various zones of the installation.

I claim:

1. A method of controlling thermal exchanges in preparation of the rawmaterials for cement before such materials enter the rotary kiln, in apreheating installation of a series of heaters through which the hotgases leaving the kiln pass successively and in which the material issuspended in the current of gas, each heater being followed by a zonefor separation of the gasmaterial mixture, the material flowing from oneseparation zone to another towards the kiln inlet and under-.

going progressive partial carbonate decomposition by heating in contactwith the gases, comprising the steps of controlling the temperature ofthe gases leaving the kiln, establishing an endothermic reaction zone inthe kiln, abruptly cooling the material in the zone of the preheaterwhere the material has reached a temperature between the temperature ofdecarburization of magnesium and the temperature of decarburization ofcalcium, and delaying decarburization of'the calcium to the endothermicreaction zone in the kiln.

2. A control method according to claim 1, including the step ofinjecting gas across the flow of material to abruptly cool the material.

3. A control method according to claim 2, including the step ofcontrolling the rate of flow of the injected gas at a fixed temperatureto control the temperature drop of the material.

4. A control method according to claim 2, including the step ofcontrolling the temperature of the injected gas at a constant rate offlow to control the temperature drop of the material.

5. A control method according to claim 2, including the step ofsimultaneously controlling the temperature and the rate of flow of theinjected gas to determine the temperature drop of the material.

6. A control method according to claim 2, including the step ofcontrolling the rate of flow of injected gas by the temperature of thegases leaving the kiln.

7. A control method according to claim 2, the injected gas beingexternal air.

8. A control method according to claim 2, the inseparating thegas-material mixture having a bottom orifice connected to a tube leadinginto a duct situated upstream of the cyclone in the direction of flow ofthe gases, the further step of injecting the injected gas into the duct,across the flow of material into the duct.

10. Apparatus for controlling the degree of preparation of the rawmaterials for cement comprising a kiln, a pre-heating installation, aseries of successive ducts in said installation for the flow of gasesleaving the kiln, each duct leading into a separation chamber, a gasoutlet orifice for each of said chambers formed by the next duct, amaterials outlet orifice for each of said chambers, a tube leading fromeach of said orifices for introducing said materials into a precedingduct, an end of said tube extending into and along the axis of the saidduct, a fan for delivering cooling gas, a gas supply casing connected tosaid fan disposed coaxially about one of the ducts, at least one orificefor injection to the interior of the duct at the outlet of the tube forthe introduction of the corresponding materials, said orifice injectinga current of cooling gas distributed uniformly about the axis.

11. Apparatus according to claim 10, the duct provided with the coolinggas injection casing being the first duct to receive the hot gasesleaving the kiln.

12. Apparatus according to claim 10, the wall of the duct with saidcasing having a plurality of uniformly spaced orifices opening into thecooling gas supply cas- 13. Apparatus according to claim 10, the lowerend of the material supply tube being open and discharging through anannular aperture and a gas dispersion cone partially closing saidaperture.

at i i

2. A control method according to claim 1, including the step ofinjecting gas across the flow of material to abruptly cool the material.3. A control method according to claim 2, including the step ofcontrolling the rate of flow of the injected gas at a fixed temperatureto control the temperature drop of the material.
 4. A control methodaccording to claim 2, including the step of controlling the temperatureof the injected gas at a constant rate of flow to control thetemperature drop of the material.
 5. A control method according to claim2, including the step of simultaneously controlling the temperature andthe rate of flow of the injected gas to determine the temperature dropof the material.
 6. A control method according to claim 2, including thestep of controlling the rate of flow of injected gas by the temperatureof the gases leaving the kiln.
 7. A control method according to claim 2,the injected gas being external air.
 8. A control method according toclaim 2, the injected gas being taken from the outlet of the pre-heatinginstallation.
 9. A control method according to claim 2, each heaterbeing a duct for the flow of the gases and the materials in suspensionand leads into a cyclone for separating the gas-material mixture havinga bottom orifice connected to a tube leading into a duct situatedupstream of the cyclone in the direction of flow of the gases, thefurther step of injecting the injected gas into the duct, across theflow of material into the duct.
 10. Apparatus for controlling the degreeof preparation of the raw materials for cement comprising a kiln, apre-heating installation, a series of successive ducts in saidinstallation for the flow of gases leaving the kiln, each duct leadinginto a separation chamber, a gas outlet orifice for each of saidchambers formed by the next duct, a materials outlet orifice for each ofsaid chambers, a tube leading from each of said orifices for introducingsaid materials into a preceding duct, an end of said tube extending intoand along the axis of the said duct, a fan for delivering cooling gas, agas supply casing connected to said fan disposed coaxially about one ofthe ducts, at least one orifice for injection to the interior of theduct at the outlet of the tube for the introduction of the correspondingmaterials, said orifice injecting a current of cooling gas distributeduniformly about the axis.
 11. Apparatus according to claim 10, the ductprovided with the cooling gas injection casing being the first duct toreceive the hot gases leaving the kiln.
 12. Apparatus according to claim10, the wall of the duct with said casing having a plurality ofuniformly spaced orifices opening into the cooling gas supply casing.13. Apparatus according to claim 10, the lower end of the materialsupply tube being open and discharging through an annular aperture and agas dispersion cone partially closing said aperture.